Success of surgical interventions performed to improve pulmonary blood flow in infants with congenital defects that interrupt the pulmonary circulation depends on the procedures and materials selected and the ability of the intervention to accommodate for patient growth. Repair options include creation of atrio- and cavopulmonary connections and implantation of cryopreserved valve homografts or extracardiac conduits. However, conduits and homografts don't grow, optimal positions for connections are controversial, and late results from clinical and autopsy studies show high incidence of conduit failure due to fibromuscular ingrowth, valvular degeneration and pseudointimal peel formation. Thus, despite significant advances, answers are still needed to many important questions concerning how to 1) prevent conduit failure, 2) maintain sufficient energy to move blood through a low pressure system, 3) appropriately distribute flow to both lungs, and 4) prepare for reasonable growth. Answers to these general questions require answers to basic questions regarding relationships between fluid mechanics and the involved geometries: How do 1) conduit design and shape affect shear stresses, secondary flow and flow separation, 2) curves, kinks, pouches and bends affect energy losses, 3) anastomotic geometries influence flow distribution? This study will address pertinent issues through a series of in vivo, in situ, and computational experiments. (A) In the in vivo phase, hemodynamic studies will be performed in 1-month and 3-month old lambs before and implanting an extracardiac shunt or establishing an atrio- or cavopulmonary connection. The importance of respiration and atrial contractions to blood transport will be evaluated. (B) Silicone rubber casts of the vena cava connections, right heart and proximal pulmonary arteries will be made in situ. (C) In vitro studies will be performed in various flow through models, velocity patterns visualized using dye injections and laser light and velocity measured with laser and pulsed Doppler devices. In vitro studies provide flexibility not available in vivo; conduit design features will be tested in a range of hemodynamic and cardiopulmonary geometric conditions. (D) Casts made in situ will be CT scanned such that computerized 3D images can be reconstructed and key features extracted. The computerized images will provide the basis for defining geometry for stereolithographic, anatomically correct flow-through models and for computation studies. Finite element techniques for simulating pulmonary blood flow will be developed, expanding the flexibility provided by the in vitro studies, including the ability to project for expected growth. (E) MR images and CT scans of Fontan patients will be used to generate comparable flow-through and finite element models for in vitro and computational studies. unification of results obtained should help identify causes of graft failure and assist surgeons in selecting interventions to establish optimal pulmonary blood flow patterns and make appropriate allowances for patient growth.